A polyester elastomer is a specific polymer having both chemical resistance and flexibility. Polyester elastomers are inferior in thermal resistance in comparison to fluorine rubber or the like and for this reason a fluorine rubber, which is expensive but can be used at a temperature of 150.degree. C. or more, generally should be used for purposes under a high temperature of from 120.degree. to 150.degree. C.
It is well known in this field of art that a flammable polymer can be made flame retadant by adding thereto a flame retardant, and that the anti-heat deformation properties of a polymer can be improved by crosslinking using electron beam irradiation.
However, even if the polyester elastomer is made flame retardant and is crosslinked by the above procedures, its use under a high temperature for a long period of time is restricted, unless its anti-heat aging properties can be improved. That is, even if the elastomer is crosslinked, the polymer molecules are broken due to oxidation deterioration upon long time use at an elevated temperature, and thus the characteristics of the elastomer (e.g., the tensile strength) is deteriorated.
It has been proposed to improve the anti-heat aging properties of a flame retardant polyester elastomer by adding a high molecular weight hindered phenol type antioxidant, a chlorine flame retardant, and a polyfunctional monomer to a polyester elastomer which is followed by crosslinking using electron beam irradiation (as described, e.g., in Japanese Patent Application (OPI) No. 163953/86) (the term "OPI" used herein means a published unexamined Japanese Patent Application). However, this method still has the disadvantage that the tensile strength and the electric properties, particluarly the volume resistivity, of the resulting polyester elastomer are not satisfactory.
As an example of a polyester elastomer containing both an antioxidant and a light stabilizer, it has been proposed in U.S. Pat. No. 4,524,165 to add a hindered phenol type antioxidant and a hindered amine type light stabilizer to a polyester elastomer so as to improve its weather resistance. However, the thermal resistance and the electric properties cannot be improved by this method.